Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements - Notes (Details)

v3.8.0.1
Fair Value Measurements - Notes (Details) - Notes - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
2033 Senior Notes $ 32,968 $ 45,205
Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
2033 Senior Notes 32,968 45,205
Carrying Value    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
2033 Senior Notes $ 29,160 $ 26,965